1. Field of the Invention
This invention relates to supported or bound heteropoly acid catalyst compositions, a method of making such catalyst compositions and a process for the oxidation of unsaturated aldehydes, such as methacrolein, to unsaturated carboxylic acids, such as methacrylic acid, in a vapor phase reaction using such catalyst compositions.
2. Description of the Prior Art
Various catalysts are known for the gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids. Included are molybdenum-based mixed metal oxides compound which can contain oxides of metals such as phosphorus, arsenic, cesium, rubidium, cobalt, nickel, iron, chromium, antimony, tellurium and silicon in addition to molybdenum. These same metals and others may occur in heteropoly acid compounds as metal oxide clusters forming heteropolyoxoanions in acid form instead of simple metal oxides. Heteropoly acid compounds are also known as catalysts for the gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids. Heteropoly acid compounds have a central metal atom surrounded by a framework of other metal atoms connected to each other and the central metal atom through oxygen atoms. The central metal atom is different (“hetero”) from the framework metal atoms.
U.S. Pat. No. 3,998,876 discloses a catalyst of a heteropoly acid compound containing phosphorus, molybdenum, arsenic, at least one of vanadium, tungsten, copper, iron, manganese or tin, at least one of lithium, sodium, potassium, rubidium or cesium and ammonium groups in the form of a salt of the heteropoly acid. The examples of the nonsupported catalyst were shown to have a higher degree of conversion at comparable selectivities compared to a catalyst supported on an alumina/silica carrier.
U.S. Pat. No. 4,320,227 discloses a heteropoly acid catalyst containing molybdenum, vanadium, phosphorus, one or more of copper, tin, thorium, aluminum, germanium, nickel, iron, cobalt, zinc, titanium, lead, rhenium, zirconium, cerium, chromium, bismuth or arsenic, and one or more of potassium, rubidium, cesium or thallium. For improvements in thermal stability and catalyst life and increase in yield of methacrolein and methacrylic acid a suitable carrier, such as silicon carbide, α-alumina, aluminum powder, diatomaceous earth or titanium oxide, can be used. Active carriers which react with heteropoly acids are not preferable.
U.S. Pat. No. 5,191,116 discloses a heteropoly acid catalyst containing molybdenum, vanadium and/or copper, phosphorus and/or arsenic, at least one of an alkali metal, such as lithium sodium, potassium, rubidium, and cesium, an alkaline earth metal, such as magnesium, calcium, strontium and barium, or thallium and at least one of silver, zinc, cadmium, titanium, zirconium, niobium, tantalum, chromium, tungsten, manganese, iron, cobalt, nickel, boron, aluminum, germanium, rhodium, tin, antimony, bismuth, selenium, tellurium, yttrium, lanthanum, cerium, praseodymium and neodymium. The catalysts may be carried on a carrier, such as silica, α-alumina, titania, zirconia, diatom earth, silica alumina, water soluble silica sol and silicon carbide. Inert carriers having a vast plurality of macropores and high porosity are preferred. One working example used a carrier of silica.
Heteropoly acid compounds which are active catalysts generally lack thermal stability. High temperatures will decompose the heteropoly acid structure. Heteropoly acid compounds also generally lack mechanical stability. Their framework structure is inherently fragile and lack of mechanical and thermal strength is a typical characteristic of heteropoly acid compounds. Supporting or binding the heteropoly acid compound to improve mechanical and thermal strength can result in decreased catalyst performance. It would be advantageous to have a supported or bound heteropoly acid compound with increased mechanical and thermal strength without loss of catalyst performance.
One solution to address thermal stability is found in U.S. Pat. No. 5,618,974 which discloses a catalyst for producing methacrylic acid by oxidation of methacrolein, isobutyl aldehyde or isobutyric acid which contains (A) a composite oxide which may be a heteropoly acid of molybdenum, phosphorus, at least one element of arsenic, antimony, germanium, bismuth, zirconium, cerium or selenium, at least one element of cooper, iron, chromium, nickel, manganese, cobalt, tin, silver, zinc, palladium, rhodium or tellurium, at least one of vanadium, tungsten, or niobium and at least one of alkali metals, alkaline earth metals or thallium and (B) a solid acid having acid strength (Ho) of no more than −11.93.
Supported heteropoly acid compounds are used in processes other than gas phase catalytic oxidation of unsaturated aldehydes to unsaturated carboxylic acids.
U.S. Pat. No. 5,990,348 discloses a process for converting alkanes, such as propane or isobutane, to unsaturated carboxylic acids, such as acrylic acid or methacrylic acid, with an oxidizing agent and a heteropoly acid supported on an insoluble polyoxometallate salt, both acid and salt having a polyhedral cage structure or framework. The support can be a cesium salt of a heteropoly acid. The heteropoly acid is water soluble.
U.S. Pat. No. 6,043,184 discloses a process for converting alkanes, such as propane and isobutane, to unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, with an oxidizing agent and a heteropoly acid supported on a porous cesium polyoxometallate salt. The heteropoly acid is water soluble.